1. Field of the Invention
The present invention relates in general to a method for obtaining particulate calcium carbonate and methods for using same, and more particularly, to a method for obtaining particulate calcium carbonate having an average particle size less than about 12 microns from a pulp mill and further having a chemical composition and/or purity which substantially inhibits the fusing together of the calcium carbonate particulates.
2. Background of the Art
Limestone, which is primarily calcium carbonate, has been quarried and processed for a wide variety of uses pre-dating the construction of the pyramids of Ancient Egypt. The direct use of limestone and the conversion of limestone to quicklime has continued unabated since that time for use in the construction of buildings and roads, as well as for glass formation and the purification of metals. The advent of the industrial and technical revolutions has continued the need for high quality calcium carbonate. However, the increased need for calcium carbonate has brought with it a demand for calcium carbonate particle sizes below that which can be accurately measured by a standard sieve analysis. Oftentimes, the particulate size requirement is only a few microns in diameter.
The use of calcium carbonate in agricultural settings and manufacturing applications is also well known in the art. Since World War II, increasing amounts of calcium carbonate materials have been spread on the soil of farms (hereinafter referred to as “liming”) as a method of increasing the productivity of the soil and aligning the soil pH closer to neutral. In fact, the direct application of calcium carbonate to soil, is the greatest single agricultural use of calcium carbonate. Its use in agricultural applications during the 1940-1970 time span accounted for approximately 70-80% of the tonnage of calcium carbonate produced. For example, in Willamette Valley, Oreg., over 150,000 tons of limestone products are used each year for soil neutralization thereby increasing the yields of a number of products including grasses for seed.
Traditionally, the calcium carbonate material has been spread by self-unloading dump or tank-type trucks and the calcium carbonate has been applied to the land by plowing about half of the calcium carbonate under the soil and harrowing in the remaining half. More recently, the soaring price of fertilizer has made the spreading of particulate calcium carbonate an attractive and inexpensive option for farmers and other agricultural users. Indeed, it has been found that the preliminary treatment of agricultural plots with calcium carbonate is a prerequisite in order to reap the full value from such costly fertilizers.
The use of calcium carbonate is varied across a wide spectrum of applications. For instance, a preponderance of crops and plants grow most profusely under neutral to slightly acidic conditions. Thus, acidic soil in the pH range of 3.5-6.0 can be made more fertile and productive for many crops by neutralizing soil acids. Also, the essential plant nutrients, calcium and magnesium, are supplied directly to the plants to support plant growth. Through liming, microbiological activities in the soil are stimulated, thereby liberating other available plant nutrients from the soil organic matter. Indirectly, liming increases organic matter in the soil by fostering larger and more prolific growth. Greater volumes of roots and plant residues are retained in and on the soil and the earthworm population generally increases as the pH of the soil is elevated up to neutral.
Numerous problems have made the agricultural application of calcium carbonate incomplete: for instance, calcium carbonate of sufficient size and surface area is expensive to obtain and cannot be produced from larger sized calcium carbonate economically. Although liming has become a requisite in the agricultural industry, the application of calcium carbonate through liming has been partially ineffective, time intensive, and potentially over applied. Furthermore, the application of powdered calcium carbonate directly to the soil is ineffective, as well, because it tends to be blown away by the wind and requires lengthy treatment times to reach the desired soil pH level.
The numerous problems of the agricultural use of calcium carbonate are mirrored and amplified in the use of calcium carbonate for flue gas desulfurization for the reduction of acid rain, in the power industry. Although low sulfur coals have been utilized in order to reduce sulfur dioxide produced, thereby postponing the installation of expensive scrubbers, tightening environmental regulations will soon force power plants to also use flue gas desulfurization “scrubbing” techniques. However, the size of the calcium carbonate currently available for use, is too large, thereby leading to an substantially ineffective scrubbing process.
The dominant process for chemical pulping in the paper industry is the alkaline “Kraft” process which uses sodium hydroxide and sodium sulfide as the primary chemical constituents. In order to make the Kraft pulping process economically feasible, the chemicals are regenerated in a series of steps, including: 1) washing the spent chemicals and digested wood substance out of the “pulp” and collecting the resultant “weak black liquor” in large tanks; 2) evaporating the liquor in preparation for burning in a Kraft recovery boiler which produces steam for energy recovery and molten “smelt” for chemical recovery, wherein the smelt drops into a tank where it is mixed with water to form “green liquor” which contains sodium carbonate and sodium sulfide; 3) the sodium carbonate is reconverted to sodium hydroxide by using calcium oxide wherein the calcium oxide is converted into a finely divided calcium carbonate called “lime mud” suspended in a regenerated pulping liquor; and 4) the calcium oxide is regenerated by burning the lime mud in a lime kiln. Before the lime mud can be burned in a cost-effective way, however, the lime mud must be separated from the regenerated pulping liquor and washed. After intensive washing and denaturing steps, the lime mud contains primarily calcium carbonate with trace amounts of calcium hydroxide and sodium hydroxide. The calcium carbonate in the lime mud ranges in size from less than 1 to greater than 120 microns.
The regeneration of the chemicals in the Kraft pulping system, however, is not entirely effective. Typically, unreactive contaminants come from: (1) The wood used to make the pulp; (2) Corrosion and erosion of piping and equipment; (3) sulfur compounds from the pulping liquors; and (4) sulfur gasses burned in the rotary lime kiln. A residual level of contaminants in the lime mud results in a reburned lime having decreased causticizing efficiencies which translates into higher energy costs throughout the Kraft pulping process. Also, since the lime kiln is often the bottleneck to higher pulp production rates, contaminants in the reburned lime can decrease overall pulp production and concomitantly increase energy costs. If the pulp mill also has a bleach cycle, the contaminants lower the brightness control of the pulp, thereby increasing the bleaching costs of the pulp production system.
Thus it is an object of the present invention to provide a method of obtaining a particulate calcium carbonate having an average particle size less than about 12 microns from a pulp mill.
It is another object of the present invention to provide a method of optimizing the operation of the recausticizing cycle in a pulp mill thereby reducing the energy costs throughout the recausticizing cycle and maximizing pulp production.
It is a further object of the present invention to provide a method of applying a particulate calcium carbonate having an average particle size less than about 12 microns to a variety of applications wherein the size of the calcium carbonate particles is of concern.
These and other objects of the present invention will become apparent in light of the present Specification, Claims, and Drawings.